yellow leaves? first time indoor grower. help!?

so, i got together some seeds that i collected from good strains in the past, germinated them myself, planted them and put them under lights for 24 hours. let them veg until about 40cm high. the lights in their veg state were just 200w tungsten because i couldn't get lights until recently. 24 december i bought one 400w high pressure sodium, transplanted them into same soil with added vermiculite, coco peat & vermiculite for better water retention. they're growing in a cupboard with a fan and decent ventilation. there are six plants, 5 are female, one still hasn't shown signs, think it might be a maf? let them chill for one more day under 24 hour lights and then changed light cycle to 12/12 for budding. mostly they're doing good, but one plant in particular is going quite yellow and i don't know why. they're all in the same soil mix and they've definitely got enough light. i thought it might be soil pH but they're all in the same soil??? why only one plant? i'll post photos asap, but does anyone have any suggestions? would be greatly appreciated!

thanks! :jointsmile:

Sometimes you just get a sickly plant. If all the other plants are doing well and you doing the same thing to each plant, that shows that there may not be a problem with what your doing but just a sickly plant. A little TLC and she'll reward you with some bud im sure. GL:hippy: